[FIXED JENKINS-11577]

Traditionally, matrix axis values are only exposed as "build variables", and it was up to individual build steps and others to treat them equally like environment variables.

However, in practice, this distinction between environment variables vs build variables didn't serve any useful purposes, and the down side (of some plugins only expanding env vars in the ${VAR} notation and not build variables) was probably bigger than whatever benefit this distinction was supposed to bring.

In this fix, we are exposing matrix axis values also as environment variables. This will make them recognizable from plugins that only expand environment variables, such as parameterized trigger plugins as indicated in the original bug report.

This was motivated by pull request #701 but fix was made differently.